Small-angle scattering data from type Ia cGMP-dependent protein kinase (PKG) titrated with cGMP (1 through 13 molar equivalents) shows a large conformational change induced by cGMP-binding. PKG is a dimer with a regulatory and catalytic domain within each monomer component. There is one 'slow' and one 'fast' cGMP binding site on each monomer. A P(r) analysis of the scattering data indicates that the conformational change involves a shift of the molecular mass away from the center of the molecule, possibly due to the movement of the regulatory domain away from the catalytic domain. The radius of gyration of PKG increases dramatically by 30% from 45.4 to 59.4 E upon addition of cGMP. It appears that the 4 cGMP binding sites per PKG may only need to be partially saturated to see the full confromational change. Further study is underway to characterize the exact role of the slow and fast cGMP-binding sites.